The present invention generally refers to the manufacturing process of paper, paperboard, cardboard and similar products, such as cellulose, fiber cement, and others. The retention and drainage systems aim at improving the drainage (the capacity to eliminate water) and the retention of fines and fillers, which are part of the paper sheet.
These systems have evolved, coming from one retention and drainage agent to two and more components used today. Each one of them shows advantages and disadvantages, while the systems most used are the ones based on microparticles.
The benefits expected from a retention and drainage system are the following: higher production (ton/hr); lower production costs; lower energy consumption; better stability and reliability of the production systems; less acidity in the system due to the decrease of aluminum sulphate and consequently fewer corrosion problems; better formation of the paper sheet (seen against the light); less porosity of the paper; and less humidity of the paper.
Traditionally the paper machines have worked with retention agents, such as: natural polymers such as starch; synthetic polymers such as cationic, anionic, amphoteric and non-ionic polyacrylamides; and polyethylene oxide.
These products have the characteristic to “retain” a higher number of solids in suspension (fines and fillers) than the stock on its own without these additives.
Therefore, these days it is common practice in the paper manufacturing industry to use stock retention and drainage aids.
In the prior art several stock retention and drainage aids are known, such as the polyacrylamides (PAM), polyethylenimines (PEI), polyamides and polyamines, mainly.
Thus, for example, in the U.S. Pat. No. 3,052,595 (the disclosure of which is incorporated herein by reference) the use of polyacrylamides with filler is described and it is stated that advantageous results are obtained when bentonite provides 1 to 20% by weight of the mineral filler. In the British Patent No. 1.265.496 it is described how polyacrylamides are used to retain organic filler and cellulosic fines but that critical conditions have to be observed for successful operation, and particular modified acrylamides are described.
In the German Patent No. 2262906 (the disclosure of which is incorporated herein by reference) it is proposed to improve the dewatering of cellulosic slurries by adding bentonite and a low molecular weight cationic polymer that serves as a poly-electrolyte. In the prior art, the amount of bentonite included in the pulp is generally between 0.02 and 2% by weight dry bentonite clay, based on dry weight of paper or pulp, and most preferably it is between 0.1 and 1%. The bentonite-type clay used in the invention may be one of the common commercially available bentonites (known as montmorillonite clays) such as “Wyoming bentonite” and “Fullers Earth”, and may or may not be chemically modified, e.g. by alkali treatment to convert the calcium bentonite substantially to alkali (e.g. sodium, potassium or ammonium) bentonite.
Another document related to the subject of the present invention is the U.S. Pat. No. 4,753,710 (the disclosure of which is incorporated herein by reference) granted to the company Allied Colloids in which a process is described according to which a cationic polymer, preferably polyethylenimine, a polyamine epichlorhydrin product, a polymer of diallyl dimethyl ammonium chloride, or a polymer of acrylic monomers is added to a watery cellulosic suspension before the last shearing stage, and bentonite is added after this shearing stage. This process allows for better retention, drainage, drying and web forming properties. The bentonite used in the said process is called “Hydrocol”.
Respectively, according to the paper production method described in the U.S. Pat. No. 5,178,730 (the disclosure of which is incorporated herein by reference), granted to the company Delta Chemicals, there is added to the stock before the shearing stage a cationic polymer, which is preferably a tertiary or quaternary amine derivative of polyacrylamide, and after the shearing stage, before the head box, there is added a natural hectorite at a weight ration 0.5:1–10:1. The method according to this patent can be used in alkaline and acid paper production processes.
In the U.S. Pat. No. 5,876,563 (the disclosure of which is incorporated herein by reference), of Allied Colloids, a cationic starch together with a cationic polymer and an anionic microparticulate material is used as the retention aid.
On the other hand, according to the Patent WO 99/14432 of Allied Colloids (the disclosure of which is incorporated herein by reference), the microparticulate aid is preferably bentonite, silica, a polysilicic acid, polysilicate microgel, or an aluminum-modified version thereof.
The use of silicate microparticles together with a cationic polymer in a retention system is described in the U.S. Pat. 5,194,120 of Delta Chemicals (the disclosure of which is incorporated herein by reference). The prevalent cation in the synthetic amorphous metal silicate was magnesium, and the polymer was preferably a tertiary or quaternary amine derivative of polyacrylamide, their weight ratio being between 0.03:1 and 30:1. By this method, retention, dewatering and formation were improved by using smaller amounts of retention aids than previously, and thus the costs were correspondingly lower.
Through U.S. Pat. No. 4,305,781, granted to Allied Colloids in 1981 (the disclosure of which is incorporated herein by reference), the use of a clay in combination with a synthetic polymer, generally based on a non-ionic linear polymer, was introduced. The combination and the adding order, first clay then polymer, increased drainage and retention. Today this is known as the dual system (two components) or microparticulate system (particle size between nanometers and micrometers).
Here below this and other systems of the prior art and their disadvantages will be described in detail:
The so-called ORGANOPOL system, described in Patent No. EP-A-0 235893 (the disclosure of which is incorporated herein by reference), consisted of two chemical products.
The first product was ORGANOSORB, which is an activated or modified bentonite, an inorganic pigment in powder, which is added as a slurry at 1.5–5% by weight in concentration, preferably 2.5% by weight to the level box. An automatic unit for the preparation of the dispersion of the bentonite in powder was used, an on-line continuous process, that is, not by batch, generating the following problems:
1. No adequate swelling is produced; that is, no de-lamination of the bentonite is produced, since the dispersion residence time in the automatic bentonite preparation equipment is very short. Therefore the bentonite does not increase the retention capacity of the colloidal and semi-colloidal particles of organic and inorganic compounds present in the stock the paper, paperboard, cardboard and other similar products are made with.
2. There are variations in the concentration of the bentonite dispersion added to the stock system the paper, paperboard, cardboard and other similar products are made with.
3. The required equipment of continuous preparation is very big, complex, difficult to operate and handle and has a very high cost, (U.S. $80.000).
4. The concentrations of bentonite above 5% by weight form very viscous, gel-like dispersions that are difficult to pump.
The second product was ORGANOPOL polymer, which is an essentially linear cationic polyacrylamide with molecular weights of more than one million, three million and higher (according to Patent No. EP-A-0 235893) is added to the thin, diluted stock, after the pressure screen.
The polymer added at this point, does not allow to reduce the size of the floccules, no smaller, tougher and more uniform floccules are obtained, and therefore there is no good formation of the paper, paperboard or cardboard sheet.
The ORGANOPOL system presents the following disadvantages: loss of whiteness, with the consequent increase of the use of optical whiteners, the adverse impact of the retention and drainage and the complex handling of the bentonite.
Afterward, in the U.S. Pat. No. 4,753,710 (corresponding to the Hydrocol process), granted in the year 1988, basically the same idea is presented, apart from the fact that the order of adding the two products to the machine is inverted, that is, first the polymer is added, and then the clay (a bentonite). The description is based on cationic linear polymers of high molecular weight before a shearing stage, and after the shearing stage the clay (bentonite) is added. Preferably at the last shearing point, which is the pressure screen.
It is important to point out that both patents, but especially the latter one, make a big distinction at the shearing point, and focuses on a specific type of polymer.
The aforementioned Hydrocol system presents several problems, including: the necessity of an on-site preparation equipment of high cost, approx. U.S. $80.000; poor hydration of the bentonite (the maximum levels are not achieved); high consumption of the product (2 to 6 kilos per ton of paper); high energy consumption; more complex operation; loss of whiteness; and excessive retention with a negative effect on the drainage.
Until today many dual systems have been presented, with the same objectives, but none have been able to overcome these inconveniences.
The existing systems are presented here below in Table 1 explaining the basic operation principles:
TABLE 1System (Company)Component 1Component 2Component 3Particol (CIBA)CationicColloidalpolyacrylamidesilicaPoliflex (CIBA)CationicInverse anionicpolyacrylamidemicro-emulsionComposil (EKA)Cationic starchPolysilicicwith highacidsubstitutiondegreePositek (Nalco)CationicHigh molecularBorosilicatecoagulantweight flocculantMosaic (Buckman)CoagulantPolyacrylamideMicroparticle(4 alternatives)
The before mentioned systems of the prior art present some advantages and disadvantages to be mentioned hereafter:
Particol has the advantage of using low application doses and its on-site preparation diminishes the possibility of losing effectiveness. The counterpart is the complex equipment at the client's plant.
Composil is a system which has good retention and drainage results, does not affect the strength properties and does not have any impact on the consumption of optical whiteners. However, the problem is that it is only effective in alkaline systems, it is expensive and good control must be kept on the dose (equipment).
Positek and Mosaic systems need more components, and therefore have more equipment and variables to control; they are more complex systems.
These and other inconveniences of the procedures of the prior art have been solved through the present invention, a detailed description of which will follow hereafter.